This invention pertains to the measurement of relative permeabilities and capillary pressures by centrifuging natural or synthetic mineral core samples. More particularly, this invention relates to an improved control and data collection system for centrifugal measurement of core samples.
The relative permeabillity and capillary pressure of mineral core samples are measured by centrifuging liquids into or out of the cores. Usually, the core is prepared in a way that it is filled with water or oil; but measurements may be made on cores containing both water and oil and sometimes measurements are made by centrifuging oil or water into a clean core. Normally, the centrifugal measurement of the core sample entails placing the liquid-bearing core sample in a core holder which communicates with a transparent catch tube. The catch tube is also called a collection tube or cup. The tube may or may not be graticulated. The catch tube may or may not be encased in an opaque outer holder which is narrowly slotted for light transmission through the catch tube. Usually, four or six coreholders are used per centrifuge. The core holders are placed in a centrifuge and spun at a fairly precise speed or at different speeds. Speeds up to 20,000 rpm are used. Most of the time the core is spun in a way that the catch tube extends outward, but there are times when an inside spin is desirable.
The major drawbacks to widespread use of the centrifuge is the difficulty of obtaining good data, especially the start of a relative permeability measurement which liquid production is rapid, and also the tedium associated with obtaining good data. It has been proposed, for example, in an article entitled "Multiphase Relative Permeability Measurements Using An Automated Centrifuge" by D. J. O'Meara, Jr. and W. 0. Lease, Society of Petroleum Engineers of AIME, SPE 12128 (19083), to use an automated centrifuge to make relative permeability and capillary pressure core measurements. This centrifugal measuring system uses a special type camera with a specially arranged photodiode array to take pictures, at pre-set times, of the catch tube whose motion is frozen by a strobe light system. A linear array camera is also called a line scan camera. A linear array is a very narrow line of very small photodiodes about an inch long. The photodiodes act as small capicitors, each storing a charge which is proportional to the amount of light impinging on its surface. Each photodiode yields one "picture element" (pixel). The diodes are on a spacing as as small 1728 photodiodes in a linear inch, that is, less than one mil in spacing. This enables measurement of a phase boundary or liquid interface to a few thousandths of the volume of the catch tube. For centrifuge measurements, a camera lens system allows this small length array to see up to about three inches. The pixel information of the camera is a series of light dependent voltages which depend on the amount of light seen by each diode. A significant difference between this earlier automated system and this invention concerns the camera operation. The camera has a clock which reads the photodiode data very rapidly in sequence. Reading of the camera pixels is accomplished with a solid state switch. The earlier automated system was synchronized on the camera clock. In a sense, the camera clock says that its is ready to send data and then it reads out the data by way of solid state switch. Thus, in one way, the earlier automated system timing was controlled by the camera. This is undesirable.
The strobe lamp is placed in line with the catch tube path and at one side of the centrifuge. Usually, the strobe lamp is laid on the bottom side of the centrifuge. The linear photodiode array of the camera is also placed in line with the catch tube path and on the opposite side of the centrifuge. Usually there is an aperature in the top of the centrifuge for the camera lens. The camera sees some background light until the strobe lamp is flashed. Even in a dark centrifuge, there is background light and dark current noise which the camera detects and accumulatively collects until the next flash. Some centrifuges have transparent plastic tops so the background light may be appreciable. This is significant because the earlier automated system requires more than one light flash per measurement. Compounding the background light by multiple flashes per measurement creates noise that obscures measurement of a liquid interface inside the catch tube.
The strobe lamp is flashed on command when the position of the catch tube is in line with the camera and strobe lamp. The strobe flash transmits light through the catch tube to the camera. The amount of light transmitted through the tube is detected by the thousand or more individual pixels in the linear array. One key factor in accurately and rapidly measuring the position of the liquid interface in the catch tube is the uniformity and intensity of the light and the number of flashes it takes to make a measurement. A typical strobe lamp uses a circular reflector with discrete flat reflector surfaces designed to cause the light to be concentrated into a circular pattern. A standard lamp causes noise and hot spots. The camera internal electronics tend to saturate during hot spots. This reduces contrast. In the previously suggested automated centrifuge system, the standard strobe lamp caused the camera to detect not only the tube image, but also a hot spot of light emanating from the strobe bulb. The earlier automated system attempted to resolve this problem by imbedding two strobe bulbs in a piece of plexiglass which placed the strobe bulbs off a direct line of sight from the camera. The plexiglass was heavily sandblasted to give a pebbly surface which was then painted reflective white. Even with this two bulb strobe lighting system, it was necessary to enhance the liquid interface by floating light-diffracting materials on the interface. For this two bulb strobe system, it also was necessary to use presicion bore glass collection tubes of square cross section. The square cross section was designed to provide surfaces of fairly uniform illumination over the tube width and to reduce the effects of variation in strobe timing that inherently occur. In the earlier automated system, standard round catch tubes acted as a lens concentrating all of the strobe light in a very narrow band along the major axis. This decreased the area over which the photodiode array of the camera was effective. If the camera was initially focused on the bright line down the middle of the tube caused by the lens effect, then any offset in the strobe light caused under-exposure of the photodiodes and hence, a complete loss of image. Moreover, it was necessary to strobe the catch tube at least three times or more to produce a good image. The number of multiple strobe flashes was set at the beginning of the run and was not changed during the run. This compounds noise problems and variations in centrifuge speed.
It would be highly desirable to have an improved computerized centrifuge measuring system that does not require multiple strobe flashes, light-deffracting materials, two strobe lights or catch tubes of square cross section.
In the earlier automated system, a computer controls the measurement and a controller only carries out the computer command as to speed and taking the image. This is undesirable. The speed control task is set on twelve bits of two parallel input-output ports which are monitored by the controller. The twelve bit speed demand of the computer is translated by a digital-to-analog converter in the controller into a fixed DC voltage. This voltage is a set point against which the controller continually compares the centrifuge speed. The speed is obtained from a pulse type signal derived from measuring the light passing through twenty holes on the circumference of a speed disk which is attached to the centrifuge motor shaft. The resulting error is translated into a voltage correction which is sent to the motor via the speed set point line. It is desirable to provide an improved and different way of controling, varying and changing the speed of the centrifuge. For example, it is desired to more accurately and frequently determine the speed and orientation of the centrifuge arms without the requirement of a computer separate from the controller.
As previously mentioned, in the previously suggested automated measuring system, the task of obtaining an image results only from strobing the collection tube of interest for a specified multiple number of three or more times. The request for an image is controlled by a computer which sends a command to the controller to take an image and have it delivered to the computer. Also as previously stated, the controller is synchronized to the camera clock and solid state switch which reads out the camera data when the camera is ready. In other words, the camera is not controlled by the controller. The controller knows when the collection tube of interest is in line with the camera by monitoring an index mark on a speed disk with twenty tube orienting holes in it. The mark shows up once per revolution of the disk and the holes are counted. When the proper hole is in line, the controller strobes this tube the required multiple number of times. Regardless of speed controls, the speed of the centrifuge varies. Even if the variation is slight, slight changes affect alignment of the strobe, catch tube and camera with each flash. Multiple flashes and slight speed changes affect image clarity and accuracy. This is compounded by use of holes to detect alignment.
In the earlier automated system, as soon as the first camera pixel starts to send its voltage to the computer the controller sends a valid data line high. This initiates an assembly language program to read pixel voltages into the computer on an analog-to-digital converter. The program reads data at exactly the camera clock frequency. It is noted that in the earlier system, the camera data is read into the computer not the controller. This ties up the computer and is undersirable.
Of the two core measurements usually conducted on cores, relative permeabiity is the more exacting. Relative permeability is measured by spinning the core holder at a high enough speed to overcome capillary effects in the core. Liquid is produced from the core into the catch tube and the amount of production is measured as a function of time. Relative permeability measurements, therefore, are concerned with the shape of a relative volume/time dependent graphical or mathematical curve. This requires rapid data taking, especially at the start of the relative permeability measurement. For increased accuracy and other reasons, it is desirable that the centrifuge system enable rapid taking of data. Rapid accurate data taking is available only if the time to take a measurement is relatively short. It is a purpose of this invention to provide such a system.
Capillary pressure experiments involve increasing the centrifuge speed at which the core is rotated in steps and measuring the steady-state liquid production at each step. Steady-state data taking can take relatively long periods of time. It is tedious and it generates a lot of unnecessary data which in the earlier automated system filled the storage capacity of the computer since the data was delivered directly to the computer.